DR. THOMSON ON CALICO-PRINTING 
24S 
nearly coincides with the line drawn per- 
pendicular to the same line a 5, from the 
point in the picture-sheet where the line 
drawn to the eye from the point in the 
ground plan cuts it, the height of the per- 
spective of the point cannot be so exactly 
found by the Rule, as the line drawn to the 
eye in the perspective view is in this case 
nearly a perpendicular line ; and the place 
where this line cuts the line let fall per- 
pendicular to a 5, in fig. 2, from the point 
in the picture-sheet, where the line drawn 
to the eye from the place of the point in 
the ground plan cuts it, is not so exactly 
marked as when these lines which mark by 
their cutting the • perspective of the point, 
eross each other in a direction nearer the 
perpendicular. When great exactness is 
. wanted in a case of this kind, it will be the 
better way to find the perspective of a 
horizontal line, parallel to the picture-sheet, 
passing through the point whose perspective, 
is wanted ; and the place where this per- 
spective line cuts the line drawn perpendicu- 
lar to the line a b, in fig 2, from the point 
in the picture -sheet where the line drawn 
from the place of the point in the ground 
plan cuts it, is the perspective of the point. 
9. When a number of circles are con- 
centric, or nearly so in the ground plan, it 
will save drawing a great many lines, if, 
after the perspective of one of them is 
drawn, a number of the points taken in the 
ground plan of the other circles to draw 
their perspectives by, are in the lines drawn 
to the eye, which pass through any of the 
other circles, or in these lines produced from 
the points in the ground plan that were used 
in drawing the perspective of the first circle : 
as in this way, the lines already drawn per- 
pendicular to the lines a b, in fig. 2, from 
the point is in the picture -sheet where the 
lines drawn to the eye cut it, will answer for 
all the circles. By taking the points in the 
ground plan of the other circles, to draw 
their perspectives by, where the lines let fall 
perpendicular to « d, in fig. 21 from the 
points in fig. 1 , that wxre used in drawing 
the perspective of the first circle, cut them, 
a deal of drawing is saved ; as one set of per- 
pendicular lines to put the heights on, will 
pass through a great many points in all the 
circles. Produce the lines perpendicular 
to « 5, if they are let fall from points on 
the side of the first circle, that is, towards 
a b. The perspective of any circle which 
stands in a plane parallel to the picture - 
sheet, is a circle. If a circle is placed in 
a plane which would run through the point 
of sight if produced, its perspective view is 
a straight line. The perspectives of circles 
having any other positions than the two now 
mentioned, are ellipses. 
Fig. a, shows an ellipse ; and fig. b, which 
is formed of two segments of a circle, is the 
way in which persons who do not under- 
stand the subject draw a circle in per- 
spective. 
ON CALICO-PRINTING. 
By Thomas Thomson, M. D., F. R. S. 
L. and E, &c. &c. 
Regius Professor of Chemistry in the 
University of Glasgow. 
( Continued from page 142.^ 
13. WHITE DISCHARGE ON MAD- 
DER-RED. — When the aluminous mordant 
already described is printed on the cloth its 
basis (alumina) becomes fixed, and ready to 
combine with whatever colouring matter may 
be subjected to its action. Another mode of 
applying the same mordant, and producing 
patterns with it, is to impregnate the whole 
cloth with it, and afterwards to print the figure 
with a substance which has the power of ren- 
dering alumina soluble in water. The cleans- 
ing processes to which all cloths impregnated 
with mordants are subjected before dyeing, 
remove that portion of the alumina which has 
been rendered soluble, and leave portions of 
the cloth in the shape of flowers, crosses, &c. 
without any material capable of fixing the 
dye-stuff. When the cloth is dyed in the way 
already described these portions remain white, 
or at least become white after the requisite 
washing. 
The substance which has been found to 
answer best for the removal of alumina and 
peroxide of iron is citric acid. Some of the 
advantages of such an acid are obvious. It 
does not corrode the cloth, though subjected 
to a considerable degree of heat. It is a 
fixed acid, with little tendency to swell or 
travel to other portions of the mordant than 
those with which it is intended to be com- 
bined; and it has the advantage over other 
vegetable acids of dissolving away very com- 
pletely all the alumina or oxide of iron, so 
that no portion ofthesemordants is retained by 
the cloth. When we consider the ease with 
which this acid is abstracted by water, from 
the insoluble citrates, we would, apriore, infer 
that it is very little adapted for this purpose 
of the calico-printer, which, in fact, it is 
found to answer better than any other. But 
the probability is that water has no such ten- 
dency to abstract it from the soluble citrates, 
as citrate of alumina, and citrated peroxide 
of iron. 
